When your plants are in red cups, take another red cup, fill it half full. Feel the weight in your hand. THAT is the weight of a fully watered Red Solo Cup planted seedling/Clone. Make sure your Red Solo Cups have FIVE, pencil-sized (3/8") drainage holes.
Water, and allow the soil to dry to the point when you stick your finger in the dirt and it is dry at 1" depth, then water again. For red solo cups this is typically every day or every other day. 4-6oz water. The goal is to water enough to give the plant what it needs, while only giving enough that the roots have to "search out" more water, growing to fill the medium in the Solo Cup.
When the roots "strike," or start coming out of the bottom of the cups, it is time to transplant to the next appropriate size container, which in my opinion should be no more than a two gallon container. This keeps roots constrained to filling out the next largest size container so you can produce the most roots.
Overwatering is the #1 killer of marijuana. Overattention is a close #2. You were overwatering, friend.
TheFuture